"for" is correct; "to" is incorrect. As for why, well, you cannot buy something "to" something else. There is no meaning of "to" that makes any sense in that context.
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ReegisSo there might be some general descriptive guidelines when to use 'for' or 'to', but eventually it all comes down to the verbRight, or the adjective or the noun or whatever there might be in the text that has some governing power over the preposition that follows.